The field of the invention relates generally to electrical fuses, and more specifically to electrical fuses having terminals that are bolted to electrical circuit conductors.
Fuses are overcurrent protection devices for electrical circuitry, and are widely used to protect electrical power systems and prevent damage to circuitry and associated components when specified circuit conditions occur. A fusible element or assembly is coupled between terminal elements of the fuse, and when specified current conditions occur, the fusible element or assembly, disintegrates, melts or otherwise structurally fails and opens a current path between the fuse terminals. Line side circuitry may therefore be electrically isolated from load side circuitry through the fuse, preventing possible damage to load side circuitry from overcurrent conditions. In view of constantly expanding variations of electrical power systems, improvements in electrical fuses are desired.
Recent advancements in electrical vehicle technologies, among other things, has presented unique challenges to fuse manufacturers. In particular, electrical power systems for conventional, internal combustion engine-powered vehicles operate at relatively small voltages, typically at or below about 48 VDC. Electrical power systems for state of the art electric powered vehicles, however, operate at much higher voltages. Electrical power systems for some electric powered vehicles may operate at voltages as high as 450 VDC or more. Operating conditions of electrical fuses in such higher voltage power systems is much more severe than lower voltage fuses commonly used in conventional, lower voltage vehicle systems. Specifically, specifications relating to electrical arcing conditions as the fuse opens can be particularly difficult to meet for higher voltage power systems, especially when coupled with an industry preference for reduction in the size of electrical fuses. Providing relatively small fuses that can capably handle high current and high battery voltages, while providing acceptable interruption performance as the fuse element operates is challenging, to say the least.
Further challenges to fuse manufactures arise from the manner of connection of the fuses to the electrical power system of an electric vehicle. For such applications, the electrical fuses are preferably bolted directly to circuit conductors in the power system. This connection method provides the best condition for any heating of the fuse in operation to be liberated into the circuit conductors and connectors. However, the bolting of the fuse directly to the circuit conductors can pose mechanical damage to the fragile features of the fuse element. Specifically, bolt torque as the fuse is mechanically connected to the circuit conductors can damage, the fragile, current-limiting features of the fuse element at a location internal to the fuse. As such damage that may occur can be difficult to predict or detect from the perspective of an installer or person servicing the electrical power system in use.
FIG. 11 illustrates an exemplary fuse element 100 capable of satisfactorily operating at higher voltages of, for example, an electrical power system associated with an electric powered vehicle. The fuse element 100 is fabricated from a generally elongated conductor body 102 having opposite ends 104 and 106. The ends 104 and 106 in the exemplary embodiment shown are generally planar elements that define connection tabs extending longitudinally from either side of a fuse element portion 108.
As shown in the example of FIG. 11, the fuse element portion 108 include a number of openings 110 formed therethrough that define current-limiting areas of reduced cross sectional area, sometimes referred to as weak spots. The number and arrangement of the weak spots may be varied in other embodiments, but are strategically selected such that the fuse element portion 110 melts, disintegrates, breaks or otherwise structurally fails when electrical current flowing through the fuse element portion 108 reaches a predetermined limit. This is referred to an “opening” of the fuse because the conductive path through the fuse element portion 110 can no longer conduct current and an open circuit condition results.
By strategically selecting the number, dimensions, and relative spacing of the openings 110 defining reduced cross sectional areas, the fuse element can reliably open in response to a specified current condition (e.g., an overcurrent condition) at one or more locations in the fuse element portion 108, typically at one or more locations adjacent to the weak spots. The weak spot openings 110 therefore affect not only the amount of current that the fuse element portion 108 can withstand, but to a large extent determine the most likely location(s) that the fuse element portion 108 will actually open in response to current conditions. While an exemplary arrangement of weak spot openings 110 is shown, other arrangements are, of course possible.
The fuse element 100 may be fabricated from a substantially planar strip of conductive material according to known techniques. Additionally, and as shown in FIG. 11, the fuse element portion 108 may be formed with a first edge 112 that is bent out of the plane of the conductive body 102. In the exemplary embodiment illustrated, the first edge 112 extends substantially perpendicularly or normal to the plane of the conductor body 102, and extends in first direction shown in FIG. 11 as an upward direction. The fuse element portion 108 may likewise be formed with a second edge 114 that is also bent out of the plane of the conductive body 102. In the exemplary embodiment illustrated, the second edge 114 extends substantially perpendicularly or normal to the plane of the conductor body 102, and extends in second direction shown in FIG. 11 as a downward direction. As such, the edges 112, 114 extend in opposite directions from one another. The formed edges 112, 114 increase the capacity of the fuse element 100 while reducing its size measured from lateral edge 112 to lateral edge 114. Other geometric variations of the fuse element portion 108 are, of course, possible with similar effect to increase the capacity of the fuse element while still providing a relatively compact size that facilitates a reduction in the size of the fuse. Alternatively, however, in another embodiment the fuse element portions 108 may be entirely planar if desired.
It is also seen that some of the weak spot openings 110 reside in part on the planar surface of the conductive body 102 and in part on the respective out of plane edges 112, 114. That is, the out of plane edges 112, 114 are also provided with weak spot openings 110. The weak spot openings in the edges 112, 114 shown are a continuation of the pattern provided on the planar surface of the conductive body 102. This need not be the case in other embodiments, however. The side edges 112, 114 need not include weak spot openings at all, or could include weak spot openings that are differently arranged from those in the main, planar surface of the conductive body 102.
Further features may provided in the fuse element portion 108 as desired. For example, time delay features, m-spot features, and other features familiar to those in the art may be provided to enhance the interruption characteristics as the fuse element 100 opens in response to selected circuit conditions. The current capacity of the fuse element 100 is determined principally by the thickness of the conductor body 102 used to fabricate the fuse element 100, the number and arrangement of the weak spot openings 110, and the dimensions of the formed edges 112 and 114. The embodiment of the fuse element 100 shown in FIG. 11 is suitable for use with an electrical power system for an electric powered vehicle operating at, for example, a voltage of about 450 VDC, and can easily withstand relatively high electrical current associated therewith while reliably providing desirable opening characteristics, in response to overcurrent conditions, to isolate load side circuitry 126 and associated power-receiving devices from the line side circuitry 124 and power supply devices.
The ends 104 and 106 of the fuse element 100, extend in the plane of the conductor body 102 and define connection tabs attachable to respective circuit conductors 116, 118 via fasteners such as bolts 120, 122. The conductor 116 may extend to and establish electrical connection with the line side power supply or circuitry 124, and the conductor 118 may extend to and establish electrical connection with the load side power receiving device or circuitry 126. Thus, when so installed, electrical current flows from the line side circuitry 124 to and through the circuit conductor 116, from the conductor 116 to and through the fuse element 100, and from the fuse element 100 to and through the conductor 118 to the load side circuitry 126. While in the example shown, the conductors 116 and 118 are shown as flat conductor bars, a variety of alternative conductors and connectors are possible to make the line and load side electrical connections. Additionally, either of the circuit conductors 116, 118 may be configured as a bus bar and receive electrical power from multiple sources (e.g. multiple batteries of the vehicle), or supply electrical power to multiple loads. In such an embodiment, a single fuse element 100 may provide fusible circuit protection to multiple loads and/or may supply power to a load or loads from more than one power supply source such as multiple storage batteries in an electrical vehicle power system.
As the bolts 120, 122 are tightened to mechanically attach the fuse element ends 104, 106 to the circuit conductors 116, 118, torque forces (indicated by the arrows F in FIG. 11) may be transmitted to the fuse element portion 108. Because of the numerous openings 110 creating the weak spots in the fuse element portion 108 at preferred locations, however, the fuse element portion 108 is a structurally fragile element compared to the fuse element ends 104 and 106. Consequently, even a relatively small torsional force F applied when the bolts 120 and/or 122 are tightened to mechanically connect the fuse element 100 to the circuit conductors 116, 118 can damage or impair the fuse element 100. Especially where the weak spot openings 110 are located, the fragile fuse element portion 108 is highly susceptible to twisting, deformation, and/or fracture that will alter its operating conditions and opening characteristics. Suboptimal fuse operation will result if such damage occurs, and in the unfortunate case wherein the fuse element portion 108 completely breaks so as to sever the conductive path through the fuse element 102 altogether, the fuse element 100 would be rendered completely inoperable.
A careful installer would perhaps be able to avoid damaging the fuse element 100 via limiting the amount of torque applied to the bolts 120, 122 as the electrical fuse is installed, but considering the potential number of different persons that may at some point install or remove a fuse in a vehicle (e.g., a manufacturer, a dealer, a service technician, and a vehicle owner all having varying amounts of training and experience) human error in advertently applying too much torque to the bolts 120, 122 seems to be inevitable across a large number of vehicles to be manufactured and periodically serviced and maintained.
Further, because the fuse element 100 is typically contained in a housing (not shown in FIG. 11), if the fuse element 100 is advertently damaged when the fuse is installed, or perhaps even removed, it is unlikely to be detected. Because fuses can be in service (i.e., connected to energized electrical circuitry) for indefinite and lengthy periods of time (e.g., years or even decades) before an overcurrent condition occurs that is sufficient to open the fuse, a suboptimal fuse can result in suboptimal circuit protection for an extended period of time. Considering the expense and sophistication of modern electric vehicles, and the many computerized controls, adequate circuit protection is paramount.
If nothing else, premature or unnecessary opening of fuses that are inadvertently damaged during installation would be an unwelcome nuisance to fuse manufacturers, vehicle manufacturers, dealers, service technicians and owners. Particularly for fuse and vehicle manufacturers, issues associated with inadvertently damaged fuses could be perceived as defects in the design and/or manufacture of the fuses or the vehicle, when in truth none exists. That is, consumer complaints may arise from perceived manufacturing issues of the fuses and/or the vehicle when instead the real problem lies, unbeknownst to the vehicle owner or those servicing the vehicle, with overly-tightened connections during installation of the fuse that, in turn, alter or affect the proper and reliable operation of the fuse as it was designed.